A magnetoresistive random access memory (MRAM) device uses the directions of magnetic moments in a ferromagnetic material to store data. The term “ferromagnetic material” as used herein refers to any material that exhibits spontaneous magnetization.
Typically, in an equilibriated or non-magnetized ferromagnetic material, the magnetic moments of atoms within a relatively large volume of such a ferromagnetic material are aligned parallelly to one another (i.e., they have the same direction) due to magnetic exchange interactions between the atoms, thereby forming magnetic domains. Although the magnetic moments of atoms within the same magnetic domains are aligned in a parallel manner, adjacent magnetic domains can be randomly oriented, i.e., the magnetic moments of atoms in adjacent magnetic domains have different directions. The boundary between two adjacent magnetic domains is typically referred to as a domain wall. In a domain wall region, the direction of magnetization gradually changes on the atomic scale. However, when a sufficiently strong external magnetic field is applied to the ferromagnetic material, all the magnetic domains in the ferromagnetic material align along the direction of the applied magnetic field (i.e., the direction of magnetization). When the external magnetic field is removed, the magnetic domains in the ferromagnetic material still remain oriented in the direction of magnetization. Application of another enough-strong magnetic field in a new direction will cause the magnetic domains to realign to the direction of the new magnetic field.
One approach to MRAM uses a magnetic tunneling junction as a memory cell The magnetic tunneling junction is formed by separating two layers of a ferromagnetic material by a thin layer of an insulating material. Each ferromagnetic material layer comprises a single magnetic domain. The magnetic domain in the first ferromagnetic material layer has a fixed direction, while the magnetic domain direction in the second ferromagnetic material layer is allowed to shift in response to an external magnetic field. Consequently, the domain direction of the second ferromagnetic material layer is either parallel or opposite to that of the first ferromagnetic material layer, which denotes a “0” or “1” state for the purpose of memory storage.
However, currently available MRAM devices can only store up to 1 megabit (Mb), which is much less than what is needed in most memory applications. In addition, each MRAM memory cell stores only one bit of data at a time, thereby significantly limiting the maximum possible memory capacity of such devices.
Therefore, there is a continuing need for improved MRAM devices with high storage density and large storage capacity.
U.S. Pat. No. 6,834,005 issued on Dec. 21, 2004 for “SHIFTABLE MAGNETIC SHIFT REGISTER AND METHOD OF USING THE SAME,” which is owned by the same entity as the present invention, discloses a memory storage device that contains a data storage track formed of a ferromagnetic wire or a strip of a ferromagnetic material.
FIG. 1A shows a partial view of such a ferromagnetic wire 100, which is homogeneously magnetized with only one magnetic domain. The arrowheads in FIG. 1A represent the magnetic moments of atoms in the ferromagnetic wire 100, which are uniformly oriented toward the right. The ferromagnetic wire 100 can be magnetized in small sections to form magnetic domains 102 and 106 of opposite directions, as shown in FIG. 1B. Such opposite magnetic domains 102 and 106 are separated from each other by a domain wall 104, within which the magnetization gradually changes from one direction to another, as shown by the arrowheads in FIG. 1B. When a current of electrons is applied to the ferromagnetic wires from right to left, as indicated by the dotted arrowheads in FIG. 1C, the magnetic domain 102 on the right expands, because the electrons are polarized by the magnetization in the domain 102. The polarized electrons have the same spin as the atoms or ions in the domain 102 and exert a force on the domain wall 104. When the density of the driving current is sufficient to overcome the resistance of the ferromagnetic material, the domain wall 104 moves from right to left.
The typical relationship between the domain wall velocity (V) and the density of the driving current is illustrated in the plot shown by FIG. 2. When the density of the driving current is below a critical current density (JC), the domain wall velocity (V) will be zero, i.e., no movement of the domain walls will be observed. When the driving current is equal to or greater than the critical current density (JC), the domain walls will move at a velocity that correlates with the specific density of the driving current.
The above-described ferromagnetic wire can therefore function as a data storage track, where information can be stored therein as the magnetic domains. An electric current can be used to effectuate the movement of such magnetic domains and the associated domain walls along the data storage track in the direction of the electron flow. When the magnetic domains and domain walls are moved past a reading device, information can be read from the data storage track. Similarly, when the magnetic domains and domain walls are moved past a writing device, information can be written into the storage track.
The memory storage device disclosed by U.S. Pat. No. 6,834,005 can be used to store numerous bits of data (i.e., on the order of 100 bits or more). Consequently, a small number of magnetic elements can be used to store a very large amount of data, which have important applications in various electronic devices, such as digital cameras, personal digital assistants, security devices, memory sticks, removable storage devices, etc.
There is a continuing need to improve the memory storage device disclosed by U.S. Pat. No. 6,834,005. More specifically, there is a need for precisely controlling movements of the magnetic domains and domain walls along the data storage track and avoiding deleterious drifting of the magnetic domains or domain walls, so that more accurate and reliable data reading and writing can be achieved. In addition, it would be desirable to provide a device that can be fabricated at lower cost with higher precision.